PLAGUE INFECTION IN RATS. 



By GEORGE W. McCoy, 

 Passed Assistant Surgeon United States Public Health and Marine-Hospital Service. 



The rat is a known or a suspected factor in the transmission of 

 several diseases, yet at present, and perhaps for many years to 

 come, the most immediate and pressing question that concerns us 

 is its relation to the origin and spread of plague among human beings. 

 For this reason a discussion of the reaction of these animals to 

 natural and to artificial infection with B. pestis becomes of prime 

 importance. 



Not only are rats believed to be more or less directly responsible 

 for cases of human plague in a community, but in addition, they 

 are believed to be the most frequent medium through which plague 

 is carried from one locality to another, for these animals are good 

 travelers, can live on a very meager ration, and can do without 

 water for a long time if food is available. We have found that on 

 a diet of dry grain alone a rat may live for over a month.l 



In this connection it may not be amiss to call attention to the 

 importance of the rat as an agent in conveying plague infection 

 to other rodents and especially to ground squirrels. There is every 

 reason for believing that the infection among the squirrels in Cali- 

 fornia was derived originally from rats. Wherry (16) states that 

 more rats than ground squirrels have been trapped in the squirrel 

 burrows in the vicinity of Berkeley, Cal. This shows how easy it 

 might be for rats to infect squirrels and vice versa. 



The clmtcal manifestations of plague in rats are of little impor- 

 tance. It is generally said that the plague-infected rat staggers 

 about with a drunken gait, loses fear of its natural enemies, and 

 is readily captured. Our experience with artificially infected rats 

 indicates that the animals show no marked manifestations of illness 

 until shortly before death when they become quiet, crouch in the 

 corner of the cage, and try to hide. 



It is rather surprising to observe that comparatively few plague 

 rats are found dead. In the San Francisco campaign, while no 

 accurate figures are obtainable, certainly not more than 20 per cent 



(29) 



